International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 5- Feb 2014
IM3 Comparative Analysis for Transmission External
Modulation Signals in Radio-over-Fiber System
Mili Muralidharn#, Parvin. kr. Kaushik*, Sanjay. kr. Sharma$
# *$
Krishna Institute of Engineering and Technology,
Ghaziabad, India
Abstract—In order to support the growing data traffic volumes,
radio-over fiber (RoF) is vastly used and it is the major concern of
research in today’s scenario. In RoF system, the RF signal is
centralized in control station (CS) and by incorporating optical
fiber link the transmission signal reaches to base station (BS). For
transporting signal from CS to BS external modulation technique
with optical single-sideband (OSSB) and optical double-sideband
(ODSB) is used in this paper because of its advantage over direct
modulation in terms of frequency chirp and bandwidth. In this
paper, we perform time-domain analysis of IM3 for both OSSB and
ODSB. Finally, we provide in view of high input signal power for
better comparative analysis performance of RoF system.
the antenna. Therefore, no frequency up/down conversion is
required at the various base station. By this mean, a simple and
cost-effective implementation is allowed at the base station.
Index Terms- Third-order intermodulation (IM3); Radio-overFiber (RoF); Optical single-sideband (OSSB); Optical doublesideband (ODSB); Control Station (CS); Base Station (BS).
I.
INTRODUCTION
The demands for higher capacity and data rate are increasing
tremendously and it needs access network to support high data
rate with lower cost. For that, radio-over-fiber (RoF) system is
found to be the most promising solution to achieve effective
delivery of wireless and baseband signals. In RoF system,
optically modulated radio frequency (RF) signal is transmitted
from a control station (CS) to a base station (BS) through an
optical fiber. The transmitted RF signal is then recovered with
the help of photo detector (PD) at the BS and via wireless
channel it reaches to user terminal (UT) [1]. Optically
modulated RF signal can be generated by two techniques, they
are
1. Direct modulation
2. External modulation
The first modulation scheme suffers from a laser-frequency
chirp effect which causes severe degradation of the system
performance. Here, external modulation scheme is used
because it has advantage over direct modulation in terms of
frequency chirp and bandwidth.
In Radio-over-Fiber architecture, a data carrying RF signal
with high frequency carrier signal i.e. millimeter wave signal is
forced on a light wave signal before being transported over the
optical link. Now, this optically modulated radio frequency
signal is reached at the BS via an optical fiber. By the help of
photodetector at the BS an original electrical signal is
recovered. Thereafter, it is amplified and then radiated through
ISSN: 2231-5381
Nowadays, optical fiber microcellular systems, in which
microcells in a wide area are connected by optical fiber and
radio signals are over an optical fiber link among base stations
and control stations has attracted much attention.
This is because of
1) The low loss and enormous bandwidth of optical fiber
2) The increasing demand for capacity or coverage
3) The benefits it offers in terms of low-cost base station
deployment in microcellular systems.
These are the reasons which make it an ideal choice for
realizing microcellular networks. In such a system, each
microcell radio port would consist of a simple and compact
optoelectronics repeater connected by an RF fiber optic link of
centralized radio and control equipment, possibly located at a
pre-existing macrocell site. By using RF antenna remoting
allows changes to the system frequency plan or modulation
format to be done at a central location, without the need to
modify any radio port equipment. Antenna remoting should
also simplify the provision of system features such as rapid
handover, dynamic channel assignment and diversity
http://www.ijettjournal.org
Page 232
International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 5- Feb 2014
combining. RoF systems are now being used extensively for
enhanced cellular coverage inside buildings such as offices,
shopping malls and airport terminals [6].
Where,
=
/ ,
,
are the normalized dc, bias voltage, and
switching voltage of DEMZM, respectively.
&
are the phase-shifted version of
&
, respectively.
A. Analysing IM3 for OSSB signals
We develop ossb signal by using eq (4), a 90 degree phase
shifter and DEMZM to analyse the third-order intermodulation
(IM3).
The power of the IM3 for ossb can be taken as
A dual-electrode Mach-Zehnder modulator (DEMZM) and PD
in the RoF system cause third-order modulation (IM3). IM3 is
the ‘stray image signal’ produced when the RF signal from the
two transmitter combine in the input stage of a receiver. Being
very close to the fundamental frequency component, IM3 is
considered to be the most severe. So, in this paper we perform
time-domain analysis of IM3 for both optical single-sideband
(OSSB) and optical double-sideband (ODSB) [2].
II.
ANALYSING IM3 COMPONENT ON OPTICAL LINK
Fig.1.2. is showing the architecture of RoF system in which
data are mixed with RF oscillator and optically modulated by
DEMZM in a control station. This signal is then reach to the
PD through a standard single-mode fiber (SSMF) where the
detection process is carried out. After that the signal is send to
the user terminal from the base station through wireless
channel.
Where,
= Power of IM3 for OSSB
= Frequency of input RF signal (GHz)
A = Constant
B. Analysing IM3 for ODSB signals
We develop odsb signal by using eq(4), a 180 degree phase
shifter and DEMZM to analyse the third-order intermodulation
(IM3).
The power of the IM3 for odsb can be taken as
Where,
= Power of IM3 for ODSB
= Frequency of input RF signal (GHz)
B = Constant
Where,
is the optical signal from a laser
&
are the input RF signals.
,
are the amplitudes from the LD and RF oscillator,
respectively;
,
&
are the angular frequencies of the
signals. Thus, the output signal of the DEMZM can be
expressed in eq(4)
ISSN: 2231-5381
III.
RESULT AND DISCUSSION
The simulation is based on the MATLAB software and the
Table.1 is obtained with the help of this simulation. In the
fig.2, it is found that at each operating frequency, we find IM3
detected power in OSSB as compared to ODSB. As per our
result, at operating frequency 60 GHz we have seen that the
value of IM3 power for OSSB is -128.8879db and for ODSB
is -130.1263db
http://www.ijettjournal.org
Page 233
International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 5- Feb 2014
TABLE .1 SIMULATION PARAMETERS
Sr.
No.
1.
2.
3.
4.
5.
6.
7.
Operating
frequency
in (GHz)
0
10
20
30
40
50
60
IM3 for
OSSB
IM3 for
ODSB
-134.7886
-132.0752
-130.3908
-132.8973
-133.5462
-134.6558
-128.8879
-125.7889
-138.2397
-128.726
-131.6352
-158.1571
-135.2371
-130.1263
Also, in this plot, the IM3 curve for OSSB external
modulation is approximately linear but in case of ODSB
external modulation it is drastically varying at different
frequency of input RF signal in GHz.
-125
CONCLUSION
We have performed time-domain analysis of IM3 for both
OSSB and ODSB. After analyzing and simulating the IM3
effect due to DEMZM, we have observed the proposed model
of Radio over Fiber at 60GHz mm wave operating frequency,
the third-order intermodulation (IM3) effect in OSSB is
comparatively better in comparison with other external
modulation technique, ODSB due to approximately linear
variation with increase in frequency. Thus, it is desirable and
necessary to have betterment on the IM3 power to improve the
system performance for its practical implementation. Finally,
we concluded that OSSB external modulation technique is
preferred over ODSB external modulation technique while
considering the effect of IM3 power and in future, we hope
radio over fiber will be backbone of mobile and broadband
system.
REFERENCES
[1]
-130
IM3 Detected power(dBm)
-135
[2]
-140
-145
[3]
-150
-155
[4]
-160
-165
OSSB External Modulation
ODSB External Modulation
[5]
-170
-175
0
1
2
3
4
Frequency of input RF signal(GHz)
5
6
x 10
10
Fig 2. IM3 detected power as a function of the frequency of the input RF
signal for OSSB and ODSB.
ISSN: 2231-5381
[6]
[7]
T. S. Cho., et al. “Analysis of CNR penalty of radio over fiber systems
including the effects of phase noise from laser and RF oscillator. Journal
of Lightwave Technology, 23,12 (Dec. 2005), 4093-4100.
T. Cho and K. Kim, “Effect of third-order intermodulation on radioover-fiber systems dy a dual-electrode mach-zehnder modulator with
ODSB and OSSB signals.” Journal of Lightwave Technology, 24, 20522058 (2006).
K. I. Kitayama, “Ultimate performance of optical DSB signal-based
millimeter-wave fiber-radio system: Effect of laser phase noise,”J.
Lightw. Technol., vol. 17, no. 10, pp. 1774–1781, Oct. 1999..
G. H. Smith and D. Novak, “Overcoming chromatic-dispersion effects
infiber-wireless systems incorporating external modulators,” IEEE
Trans.Microw. Theory Tech., vol. 45, no. 8, pp. 1410–1415, Aug. 1997..
U. Gliese, “Chromatic dispersion in fiber-optic microwave and
millimeter-wave links,” IEEE Trans. Microw. Theory Tech., vol. 44,no.
10, pp. 1716–1724, Oct. 1996.
H. Al-Raweshidy, “Radio Over Fiber Technologies for Mobile
CommunicationsNetworks” . Boston, MA: Artech House, 2002.
G. P. Agrawal, Fiber-Optic Communication Systems. New York:
Wiley,1997
http://www.ijettjournal.org
Page 234